1. Field of the Invention
This invention pertains to determining a geolocation for unknown emitters based on measurements of satellite signals.
2. Description of the Related Art
FIG. 1 shows a geolocation scenario 100 where an emitter 110 on the earth's surface 101 can be located using three satellites s1, s2, and s3. The satellites s1, s2, and s3 have non-zero velocities with respect to the earth. The unknown emitter 110 sends out signal 112 and it is received by the satellites s1, s2, and s3. The satellites s1, s2, and s3 relay the signal 114 to a ground station 120 or other observation point.
The ground station 120 computes the time-difference of arrival (TDOA), denoted as d21, between the signal 112 from the emitter 110 through the primary satellite s1 to the ground station 120 and the signal 112 from the emitter 110 through another satellite s2 to the ground station 120. Similarly, the TDOA of the signal 112 from the emitter 110 through the first satellite s1 to the ground station 120 and the signal 112 from the emitter 110 through a third satellite s3 to the ground station 120 is also measured, which is denoted as d31.
From a TDOA measurement and the locations of the two satellites used in the TDOA measurement, a three-dimensional curved surface can be derived that defines the possible locations of the emitter 110. The ground station 120 does not know precisely the positions and velocities of the satellites. A stationary reference emitter 130, whose position is known exactly, is used to reduce the effect of the position and velocity errors of the satellites on the part of the ground station 120 in the estimation of the emitter 110 location.
Previous theoretical work in this field, such as K. C. Ho and Y. T. Chan, “Geolocation of a known altitude object from TDOA and FDOA measurements,” IEEE Trans. Aerosp. Electron. Syst., vol. 33, pp. 770-783, July 1997 (“Ho and Chan”), which is incorporated herein by reference in its entirety, shows how to estimate the location of the emitter 110 by finding the intersection of two TDOA curves on the earth's surface. However, current techniques to find the location of the emitter 110 from two TDOA measurements assume that the two TDOA signals are acquired simultaneously. In practice, this is often not the case or not even possible. For example, the technical limitations of the equipment, including the limitations in the aperture of the receiving antennas of the ground station 120, may prevent two measurements from being acquired simultaneously. Also, in some situations, not all three of the satellites s1, s2, and s3 are able to observe the same signal from the emitter 110 at the same time.
Alternatively, the location of an unknown emitter 110 can be found through measurements of the Doppler frequency shift in the received signals 114 at the ground station 120. Because the satellites s1, s2, and s3 have non-zero velocities with respect to the earth, the received signals 114 at the ground station 120 relayed through the satellites s1, s2, and s3 will be subject to Doppler frequency shift. This causes a frequency difference of arrival (FDOA) of the signals 112 relayed through satellite pair s1 and s2, denoted as f21, and an FDOA of the signals 112 relayed through satellite pair s1 and s3, denoted as f31. From an FDOA measurement and the locations of the two satellites used in the FDOA measurement, a three-dimensional curved surface can be derived that defines the possible locations of the emitter 110. Again, the ground station 120 does not know precisely the positions and velocities of the satellites. A stationary reference emitter 130, whose position is known exactly, is used to reduce the effect of the position and velocity errors of the satellites on the part of the ground station 120 in the estimation of the emitter 110 location. The location of an emitter 110 can be estimated by finding the intersection of the two FDOA curves on the earth's surface.
Further alternatively, some have suggested that the location of an unknown emitter 110 can be found using one TDOA measurement and one FDOA measurement. However, these solutions require that the signals to generate the TDOA and FDOA measurements be acquired at the same time and that they come from the same satellite pair. These conditions are often not fulfilled in practice.
Thus, there is a need for systems and methods of more accurately determining the location of an emitter. There is also a need to accurately determine the location of an emitter where the signals from different satellites are acquired at different times. Also, there is a need for systems and methods of accurately determining the location of an emitter where the TDOA measurement and an FDOA measurement do not necessarily come from the same satellite pair.